1. Field of the Invention
This invention relates to a display unit having a coordinate input mechanism and in particular to coordinate input techniques thereof.
2. Description of the Related Art
For example, the display unit described in Japanese Patent Laid-Open No. Hei 2-255911 is known as a conventional display having a coordinate input mechanism.
According to the display unit, a drive voltage pulse of a display is detected by a pencil-like detector for finding the coordinates of the position indicated with the detector. Therefore, the display unit does not require an additional pressure-sensitive tablet or the like for inputting coordinates.
The display unit described in Japanese Patent Laid-Open No.Hei 2-255911 will be discussed.
FIG. 40 shows the configuration of the display unit.
In the figure, numeral 201 is a thin film EL panel, y0, y1, . . . , yn-1 are Y-electrodes, x0, x1, . . . . xm-1 are X-electrodes, numeral 202 is a Y-electrode driver, and numeral 203 is an X-electrode driver. Numeral 204 is a timing generator, numeral 205 is a pencil-like detector, for detecting coordinates, which will be hereinafter simply called the pen, numeral 206 is an amplifier, numeral 207 is a Y coordinate detector, and numeral 208 is an X coordinate detector. The Y coordinates or lines on a display screen of the thin film EL panel 201 are specified by the Y-electrodes and the X coordinates or columns are specified by the X-electrodes. The pixel corresponding to the X-electrode to which a selection voltage is applied, on the line corresponding to the Y-electrode to which a selection voltage is applied, on the thin film EL panel 201, goes ON (is displayed).
The operation of the display unit will be described with reference to FIG. 41.
FIG. 41 shows the timing of a drive voltage applied to the Y- and X-electrodes of the thin film EL panel 201.
As shown in FIG. 41, in the display unit, the 1-frame period required to display one screen is divided into the display and Y coordinate detection mode period, for performing display and Y coordinate detection, and the X coordinate detection mode period for detecting X coordinates. The former is a period in which voltage is applied to the X- and Y-electrodes for actual display and the latter is a period in which voltage application for display is not performed.
The operation in the Y coordinate detection mode period is as follows:
The Y-electrode driver 202 connected to the Y-electrodes y0, y1, y2, . . . , yn-1 applies a selection voltage VyON(H) or VyON(L) to the Y-electrodes y0, y1, y2, . . . , yn-1 in order, as shown in FIG. 41. On the other hand, the X-electrode driver 203 connected to the X-electrodes x0, x1, x2, . . . , xm-1 supplies a voltage corresponding to display data displayed on the corresponding pixel on the line corresponding to the Y-electrode to which the selection voltage is applied to the X-electrodes x0, x1, x2, . . . , xm-1 in synchronization with the scanning operation of the Y-electrode driver.
This operation displays one screen in the display period and the X coordinate detection mode period.
Meanwhile, if the selection voltage is applied to the line with which the pen 205 is in contact, electrostatic capacity coupling between the pen 205 and the Y-electrode y0, y1, y2, . . . , yn-1 corresponding to the line, causes a voltage pulse to be supplied via the amplifier 206 to the Y coordinate detector 207. The selection voltage VyON is applied to the Y-electrodes y0, y1, y2, . . . , yn-1 in order. If a counter or the like for counting which of the Y-electrodes y0, y1, y2, . . . , yn-1 the selection voltage VyON is applied to is provided, the Y coordinate with which the pen 205 is in contact can be detected if the counter value is stored at the timing at which the voltage pulse is input via the pen 205 and the amplifier 206.
Next, the operation in the X coordinate detection mode period is as follows:
As in the detection of the Y coordinates, a voltage pulse applied to the X-electrode x0, x1, x2, . . . , xm-1 is detected by the pen 205, thereby detecting the X coordinate indicated with the pen 205.
Then, in the display unit, the X-electrode driver 203 applies a selection voltage VxON to the X-electrode x0, x1, x2, . . . , xm-1 in order. Thus, electrostatic capacity coupling between the pen 205 and the X-electrode x0, x1, x2, . . . , xm-1 to which the selection voltage VxON is applied, causes a detection voltage pulse to be supplied via the amplifier 206 to the X coordinate detector 208.
Therefore, if like the Y coordinate detector 207, the X coordinate detector 208 is provided with a counter or the like for counting which of the X-electrodes x0, x1, x2, . . . , xm-1 the selection voltage VyON is applied to is provided, and the counter value is stored at the timing at which the voltage pulse is input via the pen 205 and the amplifier 206, the X coordinate with which the pen 205 is in contact can be detected.
According to the display unit described in Japanese Patent Laid-Open No. Hei 2-255911, the X coordinate detection mode period for detecting X coordinates is required in addition to the period in which selection voltage is applied to the X- and Y-electrodes for display.
The X coordinate detection mode period will be considered. For example, to detect an X coordinate in a personal computer operating with a frame period of 70 Hz, assuming that the resolution is 640 dots in the horizontal direction and 480 lines in the vertical direction, selection voltage VxON must be applied to the X-electrodes x0, x1, x2, . . . , x639 in order in the X coordinate detection mode. This means that a 640 selection voltage application pattern must be provided in the X coordinate detection mode period.
On the other hand, the X coordinate detection mode period is called a vertical retrace period (vertical interval). To provide good display, it must normally be suppressed to a period of about 5% of the 1-frame period. Therefore, if the 1-frame period is 70 Hz, the X coordinate detection mode period becomes about 700 .mu.s and the period in which selection voltage is applied to one of the X-electrodes x0, x1, x2, . . . , xm-1 becomes about 1 .mu.s.
To apply selection voltage to each of the X-electrodes x0, x1, x2 . . . , xm-1 in order every about 1 .mu.s, the X-electrodes need to be charged with a voltage sufficient for detection by the pen in that period. However, a high-performance X-electrode driver is required to provide such high-speed charge operation.
On the other hand, such an X-electrode driver that can operate at high speed cannot be provided at low costs, leading to a high cost display unit. Such a high-speed operation would increase power consumption of the display unit.